1. Field of the Invention
The present invention relates to a method for controlling sheet feed in a sheet-fed offset printing machine, and more particularly, to a method for controlling sheet feed in a sheet-fed offset printing machine wherein printing continues just prior to a misfed or double sheet entering the printing machine.
2. Discussion of the Related Art
In currently utilized sheet-fed offset printing machines, the sheets are removed from a feeder stack, transported over a conveyor table (suction tape table) to a stop, aligned, and then gripped by a pre-gripper and transferred to a first printing unit. In order to avoid an in-register print of the subject with reference to the printed sheet and machine damage as a result of sheets being drawn in a faulty manner, a monitoring of the sheet run by means of sensors is carried out at various locations in the above-described conveying path.
Thus, it is known that, shortly after the uppermost sheet has been lifted from the feeder stack, the imbricated sheet stream should be monitored for the presence of double sheets. For the purpose of controlling double sheets in the imbricated sheet stream, use is made, in particular, of scanning control devices. If, shortly after the individualization and after the uppermost sheet has been lifted from the feeder stack, a double sheet is detected, an immediate blocking of the stop and of the pre-gripper is implemented, as well as the stopping of the feeder by disengaging a clutch which connects the feeder to the drive of the printing machine. The last sheet drawn into the machine before detection of a double sheet is then still printed out, whereupon the throwing off of the blanket cylinders in the individual printing units is initiated in the correct sequence, that is to say in accordance with the sheet run. Likewise, the ink feed and damping solution feed to the printing plates in the individual printing units is also switched off.
A monitoring device for the feed of sheets, which is arranged directly downstream of the feeder stack in the conveying direction of the sheets and is used to detect double sheets, is disclosed by DE 2 930 270 C2. In DE 2 930 270 C2, the device has a driven transport roller and a scanning roller which is mounted so as to be movable with respect to the transport roller, the sheets being conveyed between these rollers.
The stopping of the feeder, which is initiated immediately in accordance with the method described above, the switching off of printing and the switching off of the ink feed and damping solution feed have, for the first two printing units specifically, a disadvantage associated therewith in that the ink flow and damping solution flow are interrupted abruptly following the last sheet which has run into the machine and been printed. If then, following the elimination of the cause of the disturbance (removal of the double sheet from the sheet stream), a sheet is once more conveyed into the machine, the previous inking and damping of the printing plates and of the blankets results in over-inking, which is only dissipated over several sheets, which means that rejects occur, i.e., poor quality prints.
A further disadvantage associated with the sheet conveying being stopped immediately after detection of a double sheet or misfed sheet directly after the sheet conveying downstream of the feeder stack is that the printer has to clear away all the imbricated sheets, for example, the 5 to 7 sheets which lie between the stop and the transport roller, and the printer has to remove the cause of the stoppage, that is to say the double sheet. It is precisely thin and sensitive printed materials which are often damaged during this removal operation, so that rejects may also occur at this point.